Dual mode cross-directional moisture control

ABSTRACT

A sheet manufacturing system having a steam profiling bar, a moisture profiling bar, a moisture sensor and a computer is disclosed. The computer is programmed to cause the correct amount of steam or water to be applied on a slice-by-slice basis based upon the mositure content profile data generated by the moisture sensor as compared against a preprogrammed, desired moisture content profile. The computer is programmed in such a way as to cause either steam or water, but not both, to be applied as needed to any given portion of each individual slice to achieve the desired moisture content profile. Thus, the combined application of both steam and water to the same portion of the sheet, and the nullifying effects thereof, are avoided, resulting in the use of less steam and less water while still providing full control over the moisture content profile with only one moisture sensor.

BACKGROUND OF THE INVENTION

The present invention relates to systems and methods for controlling aphysical characteristic of a manufactured material simultaneously withthe manufacturing process. More particularly, the present inventionrelates to a moisture control system and method which control themoisture content profile of paper sheet, in substantially discretelongitudinal segments, or "slices."

A typical paper manufacturing system has a "wet" end where itsmanufacturing process begins, and a "dry" end where the final product isrolled into large reels. The dry end is defined as being "downstream"from the wet end, and the wet end "upstream" from the dry end. At thewet end, a fluid, fibrous pulp mixture, referred to as "slurry," flowsfrom a headbox onto a swiftly moving, continuous, porous belt referredto as the "wire." This forms a continuous moving sheet of slurry,referred to as the "web," which begins to lose its moisture by drainagethrough the pores in the wire. The web continues downstream where, priorto leaving the wire, it passes over a vacuum box which draws moremoisture from the sheet. The vacuum within the vacuum box may begenerated in a number of ways well known in the art; for example, bydrawing air from within the vacuum box through an exhaust port with afan.

To assist the vacuum box in drawing moisture from the web, steam may beapplied to the top surface of the web by way of a "steam profiling bar"prior to the web passing over the vacuum box. By applying steam to thetop surface of the web, the moisture within the web is heated andtherefore its viscosity is reduced. This reduced viscosity causes themoisture to be more easily withdrawn from the web by the vacuum box. Thesteam profiling bar is capable of applying steam in a non-continuousmanner across the width of the web. In other words, separate steamapplicators within the steam profiling bar apply steam to substantiallydiscrete, but slightly overlapping longitudinal segments, or "slices,"across the width, or "cross-direction," of the web. Thus, more or lesssteam may be applied among the individual slices so that more or lessmoisture may be withdrawn from the individual slices, thereby allowingthe moisture content profile in the cross-direction of the web to betailored as desired.

Following the steam profiling bar and vacuum box, the continuous movingsheet leaves the wire and typically enters a series of opposing,felt-covered rolls which press additional moisture from the sheet. Theserolls form the "press section" of the paper making machine. Followingthe press section, the sheet goes through a series of heated dryingdrums. As the sheet winds through these drying drums, it maintainsphysical contact with much of the circumferential surface area of eachdrying drum. This allows the drying drums, which are filled withpressurized steam, to heat and therefore still further dry the sheet.

Part way through this series of drying drums, the sheet passes a"moisture profiling bar." This moisture profiling bar sprays the sheetwith water as desired across the cross-direction of the sheet. Similarto the steam profiling bar discussed above, this moisture profiling baris capable of applying varying amounts of water to each slice across thecross-direction of the sheet, thereby allowing the moisture contentprofile in the cross-direction to be tailored as desired.

Following the moisture profiling bar, the last of the drying drums, andpossibly additional processing, the sheet is rolled into a large reel.Immediately prior to being rolled up into the reel, the sheet willtypically pass a scanner containing several sensors. These sensors aremoved back and forth across the width of the sheet by the scanner andmonitor various physical characteristics of the sheet, such as gloss,density and moisture content. Measuring the moisture content profile atthis point indicates whether the paper just produced is either too wetor too dry. These measurements may be compared against the desiredmoisture content profile, whereupon the steam applied by the steamprofiling bar and water applied by the moisture profiling bar may beadjusted appropriately.

The purpose of the steam and moisture profiling bars is to control thefinal moisture content profile of the manufactured paper. Typically, thedesired profile will be flat. That is, it is usually desirable to have auniform moisture content across the width and length of the sheet. Paperis sold by weight and generally has a moisture content profilespecification. If the moisture content profile can be tightlycontrolled, the paper can be produced having the maximum moisturecontent, while still being within the specified limits. This results ina cheaper product, since the final product for sale contains less pulpand chemicals (expensive) and more water (inexpensive) per pound.

The prior art uses several means by which the moisture and steamprofiling bars are controlled to achieve and maintain the desiredmoisture content. One means uses the measurements from the singlescanner as discussed above to control both the moisture profiling barand the steam profiling bar. In an attempt to avoid interaction betweenthe moisture and steam profiling bars, i.e., applying both moisture andsteam to the same portion of paper sheet, the control loops for the twoprofiling bars are "de-tuned." In other words, the control loops areestablished such that one profiling bar would respond substantially moreslowly than the other profiling bar. However, this means is not entirelysuccessful since both water and steam may still be applied to the sameportions of the sheet. This is undesirable since the application ofwater would tend to nullify the effect of the application of the steam.Moreover, steam is relatively expensive to produce and apply, and suchan expense is simply wasted when the drying effects of the steam arenullified by the subsequent application of water.

A second prior art system provides for the use of a second scannerassociated with the steam profiling bar, whereby each of the profilingbars would thus have its own dedicated scanner. This second scannerwould be located at a point between the steam profiling bar and themoisture profiling bar. Each scanner would independently control its ownprofiling bar. This configuration, however, also may result in theapplication of both water and steam to the same slice. This isundesirable for the reasons stated above, and moreover, requires theadded expense and maintenance costs of a second scanner.

SUMMARY OF THE INVENTION

The present invention may operate with the typical paper manufacturingsystem, as described above, which includes a steam profiling bar, amoisture profiling bar, and a single cross-directional scanner with asensor(s) for moisture content profile measurements. The results of themoisture content profile measurements taken by the scanner's sensor areprovided to the central process control computer for the papermanufacturing machine. This computer is preprogrammed to determine themoisture content of each slice based on these measurements. Thismeasured moisture content profile is then compared to a desired moisturecontent profile (typically uniform, or "flat") which was previouslyprogrammed into the computer. The computer then controls the moisturecontent of each slice by controlling the steam profiling bar and themoisture profiling bar as follows: if a slice is too dry, the steamapplied thereto, if any, is decreased. If after another moisturemeasurement by the scanner the slice is still too dry, the steam appliedthereto is further decreased. This continues until steam is no longerbeing applied to the slice. If then the slice is still too dry, thecomputer instructs the moisture profiling bar to begin applying water tothat particular slice. If necessary, this application of water isincreased until the slice reaches a desired moisture content.

Conversely, if a slice is too wet, the moisture profiling bar isinstructed to decrease its application of water to that slice, if any.If, after another moisture measurement by the scanner the slice is stilltoo wet, the application of water is further reduced. This actioncontinues until water is no longer being applied, whereupon if the sliceis still too wet, the steam profiling bar is instructed to beginapplying steam. This application of steam is increased until the sliceachieves the desired moisture content.

Thus, either steam or water, but not both, is applied to any givenportion of a given slice. The present invention therefore provides theadvantages of requiring only one cross-directional scanner and moisturesensor, and requiring less steam and water since only steam or water,not both, is applied to any given portion of a given slice of the sheet.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view illustrating the major components in atypical paper manufacturing system as configured for a preferredembodiment of the present invention.

FIGS. 2A and 2B illustrate a flow chart for a representative computerprogram used in the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 illustrates perspectively the major components of a typical papermanufacturing system 10 as configured for a preferred embodiment of thepresent invention. Upon exiting the headbox 12, the slurry 14 impingesupon the wire 16 and begins to travel downstream. Before leaving thewire 16, the web 17 passes beneath a steam profiling bar 20 and over avacuum box 22. The web 17 leaves the wire 16 as paper 18 and enters thepress section 23 and a first set of drying drums 24. Further downstream,the paper 18 passes beneath a moisture profiling bar 26 before enteringthe last set of drying drums 28. Finally, the paper passes through ascanning assembly 30 and is rolled up into a reel 32. A central processcontrol computer 34 controls application of steam by the steam profilingbar 20, the application of water by the moisture profiling bar 26 andthe scanning assembly 30 via control cables 36, 38, 40.

At the wet end, as the web 17 nears the end of the wire 16 and passesbeneath the steam profiling bar 20, steam is applied to the individualslices 42 by way of separate steam applicators 44. The steam applicators44 may be uniformly spaced, individually controllable steam valves (notshown) mounted on a common steam conduit, and may be actuated byelectromagnetic, pneumatic, hydraulic or mechanical means (not shown).The steam profiling bar 20 receives its steam by way of a steam conduit46 from a source of steam, such as a boiler (not shown). The applicationof the steam causes the moisture within the web 17 to become warmer andtherefore have lower viscosity. This allows more of the moisture withinthe web 17 to be withdrawn by the vacuum created by the vacuum box 22below. The separate steam applicators 44 are capable of applyingselectably varying amounts of steam to each slice 42 and are controlledindividually by the computer 34 by way of a control cable 36 andassociated electromechanical transducers (not shown).

Further downstream between the first set of drying drums 24 and the lastset of drying drums 28, the paper 18 passes beneath a moisture profilingbar 26. The moisture profiling bar 26 receives its water by way of awater conduit 48 and applies water to the individual slices 42 by way ofseparate water spray applicators 50. The water applicators 50 may beuniformly spaced, individually controllable water spray valves (notshown) mounted on a common water conduit, and may be actuated byelectromagnetic, pneumatic, hydraulic or mechanical means (not shown).Similar to the steam applicators 44, these water applicators 50 arecapable of applying selectably varying amounts of water to each slice 42and are individually controlled by the computer 34 by way of a controlcable 38 and associated electromechanical transducers (not shown).

At the dry end, the scanning assembly 30 with its scanning head 52monitors the paper on a slice-by-slice 42 basis, by scanning back andforth in the cross-direction. The data obtained by thiscross-directional scanning results in a "moisture content profile"measurement which is transmitted to the computer 34 by way of a cable40.

In this preferred embodiment, the computer 34 is programmed to analyzethe moisture content profile data obtained by the scanning assembly 30and determine whether any slices 42 are either too dry or too wet. If agiven slice 42 is too dry, either less steam or more water must beapplied. If the slice 42 is too wet, either more steam or less watermust be applied.

The operation of the preferred embodiment of the system 10 of thepresent invention, as depicted in FIG. 1, is as follows: the steam andwater outputs of the steam profiling bar 20 and moisture profiling bar26, respectively, are preset to some value by the computer 34. Thispreset value may be zero or some minimal value of either steam or water,as desired. The moisture content profile data gathered by the scanningassembly 30 is then analyzed. The computer 34 then determines whethereach slice 42 is either too dry or too wet, as compared to apreprogrammed, desired moisture content profile. If a slice 42 is toodry, either less steam or more water must be applied to that slice 42.Therefore, the computer 34 first checks to see if steam is being appliedto that slice 42 via its associated steam applicator 44. If steam isbeing applied, the computer 34 commands the steam profiling bar 20 todecrease, by a predetermined incremental amount, the amount of steambeing applied by the particular steam applicator 44 associated with thatslice 42. After a time period of "t_(s) " seconds (defined below), thecomputer re-analyzes the moisture data for that slice 42. If it is stilltoo dry, the foregoing process is repeated until steam is no longerbeing applied to that slice 42. If however, the slice 42 is too dry butno steam is being applied to that slice 42, or is no longer beingapplied to that slice 42, the computer 34 then commands the moistureprofiling bar 26 to begin applying water to that slice 42, or increasethe amount of water if it is already being applied to that slice 42, bya predetermined incremental amount, via the water applicator 50associated with that slice 42. Following a time period of "t_(w) "seconds (defined below), the moisture data for that slice 42 isre-analyzed by the computer 34 to determine whether additional waterneeds to be applied to that slice 42 by the moisture profiling bar 26via its associated water applicator 50. If more water is required, thelatter foregoing process is repeated.

If, on the other hand, a slice 42 is not too dry but is instead too wet,the computer 34 first checks to see whether water is being applied tothat slice 42 via its associated water applicator 50. If so, thecomputer 34 commands the moisture profiling bar 26 to decrease, by apredetermined incremental amount, the amount of water being applied bythe particular water applicator 50 associated with that slice 42.Following a time period of t_(w) seconds, the moisture data for thatslice is re-analyzed by the computer 34. If the slice 42 is still toowet, the foregoing process is repeated, until water is no longer beingapplied to that slice 42. If however, the slice 42 is too wet but nowater is being applied to that slice 42, or is no longer being appliedto that slice 42, the computer 34 commands the steam profiling bar 20 tobegin applying steam to that slice 42, or increase the amount of steamif it is already being applied to that slice 42, by a predeterminedincremental amount, via the steam applicator 44 associated with thatslice 42. Following a time period of t_(s) seconds, the moisture datafor that slice 42 is re-analyzed by the central computer 34. If theslice 42 is still too wet, the latter foregoing process is repeated.

The predetermined incremental amounts of changes in applied steam andwater, discussed above, are based upon a priori knowledge of the desiredmoisture content profile and the effective resolutions, i.e., thesmallest selectable variations in output levels, of the individual steamapplicators 44 and water applicators 50, respectively. For example, ifthe acceptable moisture content profile allows for only small variationsin sheet moisture, then the applicators' individual resolutions shouldbe high, i.e., they should provide small selectable variations in steamand water output levels.

As shown in FIG. 1, the time periods of "t_(s) " and "t_(w) " seconds,referred to as the steam and water "dead" times, respectively, representthe time it takes for the paper 18 to travel from the steam profilingbar 20 and moisture profiling bar 26, respectively, to the sensingassembly 30. These dead times include time delays introduced by thefinite response times of the individual steam applicators 44 and waterapplicators 50. Thus, when the computer 34 orders a change in the amountof steam or water to be applied, the results of that change will not beseen by the sensing head 52 within the scanning assembly 30 until t_(s)or t_(w) seconds later, respectively.

The above-described process for measuring sheet moisture content andchanging it through the appropriate application of more or less steam orwater is done continuously for each individual slice 42 during themanufacturing process of the paper 18. This allows the moisture contentprofile of the sheet paper 18 to be tailored as desired simultaneouslywith its manufacture. Thus, when the reel 32 is removed, the paper 18thereon contains the desired moisture in the desired profile (typicallyuniform, or "flat").

FIGS. 2A and 2B, a simplified flowchart for a representative computerprogram which may be used in the present invention, illustrate the logicrepresentative of that associated with the measurement and analysis ofthe moisture content profile data, as well as the control of theapplication of steam and water by the computer 34, as described above.Other flowcharts, representing other logic, would also be within thescope of the present invention and within the abilities of those personsskilled in the art based upon the foregoing discussion.

FIG. 2A illustrates in flowchart form the initialization logic 100representative of that logic which may be used to initialize a countervariable "C," status bits "S_(c)," "W_(c)," and an elapsed timer "T_(c)" used within the computer program used by the process control computer34. The counter variable C is used to keep track of the "affected" slice42, i.e., that slice 42 which is currently being analyzed and/oroperated upon. The steam applicator status bit S_(c) is used to indicatewhether the steam applicator 44 associated with the affected slice 42has been commanded by the computer 34 to begin, cease, increase ordecrease its steam output.

Likewise, the water applicator status bit W_(c) is used to indicatewhether the water applicator 50 associated with the affected slice 42has been commanded by the computer 34 to begin, cease, increase ordecrease its water output. The elapsed timer T_(c) is used to indicatethe time elapsing after the computer 34 has commanded the steamapplicator 44 or water applicator 50 associated with the affected slice42 to begin, cease, increase or decrease its steam or water output,respectively. This allows the computer 34 to know when sufficient timehas elapsed, as compared to the steam and water dead times t_(s), t_(w),such that the moisture content data for the affected slice 42 beingmeasured by the sensing head 52 and transmitted to the computer 34represents the effect on that slice 42 caused by the modifiedapplication of steam or water.

Following the start 102 of the program, the program flow proceeds toblock 104 where the counter variable C is initialized at "1," toindicate the first affected slice 42. The program flow then proceeds toblocks 106 and 108 where the steam and water applicator status bitsassociated with the first affected slice 42 are reset to logical zeros.Program flow then proceeds to block 110 where the elapsed timer T_(c)associated with the first affected slice 42 is initialized to indicatezero elapsed time. Proceeding to block 112, the counter variable C isincremented to indicate the second affected slice 42. At block 114 theprogram compares the counter variable C to a preprogrammed number "P"which indicates the number of slices 42 to be analyzed and/or operatedupon. If the counter variable C does not yet exceed the number P ofslices 42, program flow returns to the point 116 to repeat the foregoingprogram steps of resetting the status bits S_(c), W_(c) and initializingthe timer T_(c) associated with the affected slice 42. Once the countervariable C exceeds the number P of slices 42, program flow proceeds toblock 118 where, as described earlier, the outputs of the steam andwater profiling bars 20, 26 are preset. Following this preset, programflow proceeds to the main operational portion 120 of the program, therepresentative logic for which is illustrated in FIG. 2B.

FIG. 2B illustrates in flowchart form the main operational logic 120representative of that used in the main operational portion of thecomputer program used by the computer 34. Following the initializationsequence 100 described above for FIG. 2A, program flow proceeds to block122 where the counter variable C is re-initialized at "1" to againindicate the first affected slice 42. Program flow then proceeds toblock 124 where the moisture content profile data measured by thesensing head 52, as described earlier, is inputted to the computer 34.The first time through, program flow proceeds directly through blocks126 and 128 since the status bits S_(c), W_(c) had just been reset tological zeros in the initialization sequence 100 described above forFIG. 2A. At block 130, the moisture content data for the affected slice42 is analyzed to see whether the affected slice 42 is too dry. If not,the program flow proceeds to block 132. However, if the affected slice42 is too dry, the program flow proceeds to block 134 where it isdetermined whether steam is being applied to the affected slice 42. Ifsteam is being applied, program flow proceeds to block 136 where theamount of steam being applied by the associated steam applicator 44 isreduced by a predetermined incremental amount, as described earlier, andthe corresponding steam applicator status bit S_(c) is set to a logicalone, as shown in block 138. If, on the other hand, steam is not beingapplied, program flow proceeds to block 140 where the associated waterapplicator 50 is instructed to begin or increase its application ofwater to the affected slice 42, and the associated water applicatorstatus bit W_(c) is set to a logical one, as shown in block 142.

If at block 130 it was determined that the affected slice 42 was not toodry, the program flow proceeds to block 132 where it is determinedwhether the affected slice 42 is too wet. If not, program flow proceedsto block 144 where the counter variable C is incremented by one. If,however, the affected slice 42 is too wet, program flow proceeds toblock 146 where it is determined whether water is being applied by theassociated water applicator 50. If water is already being applied,program flow proceeds to block 148 where the associated water applicator50 is instructed to decrease the amount of water it is applying to theaffected slice 42, and the associated water applicator status bit W_(c)is set to a logical one, as indicated in block 150. If, on the otherhand, no water is being applied, program flow proceeds to block 152where the associated steam applicator 44 is instructed to begin orincrease the amount of steam it is applying to the affected slice 42,and the associated steam applicator status bit S_(c) is set to a logicalone, as indicated in block 154.

Following the logical setting of the status bits S_(c), W_(c) in blocks138, 142, 150, 154, program flow proceeds to block 156 where theassociated elapsed timer T_(c) is started. Program flow then proceeds topoint 158 and block 144 where, as stated above, the counter variable Cis incremented by one.

Following its incrementation, the counter variable C is compared to thepreprogrammed number P of slices 42 in program block 160. Still beingthe first time through the program and the counter variable C onlyindicating the second affected slice 42, the program flow will proceedto point 162 and the foregoing steps will be repeated. Once the countervariable C has been incremented in block 144 beyond the number P ofslices 42, program flow will proceed from block 160 to point 164 wherefull, normal program operation will begin.

Once full, normal program operation begins, the counter variable C isonce again initialized at "1" to indicate the first affected slice 42(block 122) and the moisture content profile data measured by thesensing head 52 is re-inputted (block 124). Program flow proceeds toblock 126 where the steam applicator status bit S_(c) is checked. If alogical zero (i.e., associated steam applicator 44 had not recently beeninstructed to modify its steam output), program flow proceeds to block128. However, if a logical one (i.e., associated steam applicator 44recently instructed to modify its steam output), program flow proceedsto block 166 where the associated elapsed timer T_(c) is checked to seeif the time elapsed since the associated steam applicator status bitS_(c) was set to a logical one exceeds the steam applicator dead timet_(s). If so, the status bit S_(c) is reset to a logical zero (block168) and the elapsed timer T_(c) is re-initialized (block 170), andprogram flow proceeds to point 172. If, on the other hand, the elapsedtime does not exceed the steam applicator dead time t_(s), program flowproceeds to block 174 where the program enters a waiting mode, i.e.,program flow stays within a loop 176 until sufficient time has elapsed.Once the elapsed time, as indicated by the elapsed timer T_(c), hasexceeded the steam applicator dead time t_(s), the associated steamapplicator status bit S_(c) is reset to a logical zero (block 178).

In block 128, the program checks the water applicator status bit W_(c)associated with the affected slice 42. If a logical zero (i.e.,associated water applicator 50 had not recently been instructed tomodify its water output), program flow proceeds to block 130. However,if a logical one (i.e., associated water applicator 50 recentlyinstructed to modify its water output), the elapsed timer T_(c) ischecked to see if the time elapsed since the logical setting of thewater applicator status bit W_(c) exceeds the water applicator dead timet_(w) (block 180). If so, the water applicator status bit W_(c) is resetto a logical zero (block 182), the elapsed timer T_(c) is re-initialized(block 170) and program flow proceeds to point 172. If, on the otherhand, the elapsed time does not exceed the water applicator dead timet_(w), program flow proceeds to block 184 where the program idles in aloop 186 until sufficient time has elapsed. Once the water applicatordead time t_(w) has been exceeded, the associated water applicatorstatus bit W_(c) is reset to a logical zero (block 188).

After the appropriate, associated status bit S_(c), W_(c) has been resetto a logical zero in block 178 or 188, the elapsed timer T_(c) isre-initialized (block 190) and new moisture content profile data isinputted (block 192). It is necessary for new input moisture contentprofile data to be entered at this point because, as described above,the program has had to wait for sufficient time to elapse such that theeffects of the modification of steam or water application may bemeasured by the sensing head 52. Once this new moisture content profiledata has been inputted, program flow proceeds to point 194 and on toblock 130.

Once program flow has proceeded to blocks 130 and 132, the program flowdescribed above for block 130 through point 158 is repeated. Until thecounter variable C exceeds the preprogrammed number P of slices 42, asdetermined in block 160, the foregoing program flow beginning with block126 is repeated. Once the counter variable C exceeds the number P ofslices 42, program flow beginning with block 122 is repeated, i.e.,analysis and treatment by steam or water of the slices 42 is repeated,beginning with the first affected slice 42.

The foregoing description is to be considered as merely exemplary andnot limiting in any way with respect to the present invention and anyembodiment thereof. Thus, the present invention is not necessarilylimited to the control of moisture in paper, but may extend, forexample, to the control of any physical property or characteristic inany material which can be affected in opposing senses with two spaceddevices and sensed by a single sensor downstream of the two spaceddevices. Therefore, the specific scope and subject matter of the presentinvention is to be determined according to the claims.

What is claimed is:
 1. A method for controlling the moisture content ofa sheet, comprising the steps of:(a) measuring the moisture content ofthe sheet; (b) comparing the measured moisture content with apredetermined moisture content; (c) if the moisture content is greaterthan the predetermined moisture content, then decreasing wetting of thesheet at a first location and subsequently increasing drying of thesheet at a second location until the predetermined moisture content isachieved without wetting and drying the same portion of the sheet; and(d) if the measured moisture content is less than the predeterminedmoisture content, the decreasing the drying of the sheet at the secondlocation and subsequently increasing the wetting of the sheet at thefirst location until the predetermined moisture content is achievedwithout wetting and drying the same portion of the sheet.
 2. The methodof claim 1, wherein the first location is spaced from the secondlocation.
 3. A method as in claim 2, wherein wetting the sheet isachieved by applying water to the sheet, drying the sheet is achieved byapplying steam to the sheet and the amounts of water and steam appliedto the sheet are varied incrementally, the moisture content of the sheetbeing measured after each incremental variation in the application ofwater or steam.
 4. A manufacturing apparatus including a pathway for asubstantially continuous paper sheet moving from an upstream position toa downstream position along the pathway, the apparatus comprising:afirst device, including a steam applicator, disposed at a first locationalong the pathway, for applying steam to a first surface of the sheet,for affecting the sheet moisture content when the first device is inoperation, and including a vacuum device for applying a vacuum to asecond surface of the sheet opposite the first surface; a second device,disposed at a second location spaced along the pathway from the firstlocation, for affecting the sheet moisture content when the seconddevice is in operation in a manner tending to counteract the effect ofthe first device; a sensor, disposed downstream along the pathway fromthe first and second devices, for sensing the sheet moisture content andgenerating a signal indicative thereof; and a controller, coupled to thefirst and second devices and to the sensor, for adjusting the first andsecond device in accordance with the signal, so that either the firstdevice or the second device operates.
 5. An apparatus as in claim 4,wherein the second device includes a water applicator for applying waterto a surface of the sheet.
 6. A sheet manufacturing system, including asubstantially continuous paper sheet moving along the pathway from theupstream position to the downstream position, comprising:a sheet pathwayfor directing the sheet from an upstream position to a downstreamposition along the pathway; a first device, including a steamapplicator, located at a first position along the pathway, for applyingsteam to a surface of the sheet, thereby affecting the sheet moisturecontent when the first device is in operation; a second device,including a water applicator, located at a second position along thepathway spaced from the first position, for applying water to a surfaceof the sheet, thereby affecting the sheet moisture content when thesecond device is in operation in a manner tending to counteract theeffect of the first device; a sensor, including a moisture sensor,located downstream along the pathway from the first and second devices,for sensing the sheet moisture and generating a signal indicativethereof; and a controller, coupled to the first and second devices andto the sensor, for adjusting the first and second device in accordancewith the signal, so that either the first device or the second device isin operation.
 7. An apparatus for controlling a physical characteristicof a sheet of material moving from an upstream position to a downstreamposition along a pathway, the apparatus comprising:a first device,disposed at a first location along the pathway, for affecting a physicalcharacteristic of the material at a plurality of cross-directionalsections when the first device is in operation, the first deviceincluding a steam applicator for applying steam to a first surface ofthe sheet and a vacuum device for applying a vacuum to a second surfaceof the sheet opposite the first surface; a second device, disposed at asecond location along the pathway, for affecting the physicalcharacteristic at the plurality of cross-directional sections when thesecond device is in operation in a manner tending to counteract theeffect of the first device; a sensor, disposed downstream along thepathway from the first and second devices, for sensing the physicalcharacteristic at the plurality of cross-directional sections andgenerating a signal indicative thereof; and a controller coupled to thefirst and second devices and to the sensor, the controller for adjustingthe first and second device in accordance with the signal, so thateither the first device or the second device is in operation.
 8. Anapparatus for controlling a physical characteristic of a sheet ofmaterial moving from an upstream position to a downstream position alonga pathway, the apparatus comprising:a first device, disposed at a firstlocation along the pathway, for affecting a physical characteristic ofthe material at a plurality of cross-directional sections when the firstdevice is in operation; a second device, including a water applicator,disposed at a second location along the pathway, for applying water to asurface of the sheet, thereby affecting the physical characteristic atthe plurality of cross-directional sections, when the second device isin operation in a manner tending to counteract the effect of the firstdevice; a sensor, disposed downstream along the pathway from the firstand second devices, for sensing the physical characteristic at theplurality of cross-directional sections and generating a signalindicative thereof; and a controller coupled to the first and seconddevices and to the sensor, the controller for adjusting the first andsecond device in accordance with the signal, so that either the firstdevice of the second device is in operation.